With the increasing down-scaling of integrated circuits, the sizes of metal-oxide-semiconductor (MOS) field-effect transistors (FET) have been increasingly reduced. As a result, existing semiconductor devices are formed with a minimum size in a half-micron region or in a sub-half micron region. At this stage of development, efforts are being made to bring the sizes of MOSFETs down to a quarter micron or a sub-quarter micron.
With such small sizes, the electric characteristics of the MOSFETs are likely to be degraded because of short-channel effects or hot-carrier effects, thereby seriously influencing the reliability of the MOSFETs. On the other hand, high-speed operations and low power consumption are also desired. In order to reduce the degradation caused by hot-carrier effects and/or short-channel effects, and at the same time improve the drivability, MOSFETs having asymmetric impurity concentration profiles have been proposed.
FIG. 1 illustrates a conventional asymmetric MOSFET, which includes gate dielectric 4 on semiconductor substrate 2, and gate electrode 6 on gate dielectric 4. The MOSFET further includes source/drain regions 10 and source extension region 8. On the drain side, no source/drain extension region is formed. Typically, in order to form the asymmetric structure, a photoresist is formed to mask the drain side of the MOSFET, then the source extension region 8 is formed by an implantation.
Asymmetric MOSFETs have high drive currents, and can be scaled down more than symmetric MOSFETs. Therefore, it is preferred to use asymmetric MOSFETs to replace symmetric MOSFETs in integrated circuits. In typical integrated circuits, most of the symmetric MOSFETs can be replaced by asymmetric MOSFETs. Only a small percent of symmetric MOSFETs cannot be replaced. Accordingly, the integrated circuits have to have mixed symmetric and asymmetric MOSFETs. Such a mixed scheme not only results in extra masks and implantation steps, but also creates difficulty and increased cost in the modeling.
Accordingly, what is needed in the art is a MOSFET structure and methods for forming the same that may incorporate asymmetric MOSFETs to take advantage of the benefits associated with improved performance while at the same time overcoming the deficiencies of the prior art.